of all, I have to say that the usual way of reading the jetting by the
colour of the sparkplug insulator nose is wrong. The colour of the
insulator is affected by the jetting and the heat range of the
plug. So an improper heat range sparkplug with correct jetting or an
correct sparkplug with wrong jetting will produce an colour that will
misslead you in reading the jetting. Let's make some examples;
Your jetting is ok, and your sparkplug is of an relatively hot heatrange.
The colour of the insulator nose will result of a bright gray - white
colour. It will make you think that your jetting is lean and make you
enrich the jetting. The result is loss of power or/and fouled sparkplug..
Your jetting is lean and your sparkplug heatrange is cold.The colour of
the insulator nose will result brown, making you think that everything is
ok. But you may end up overheating.
facts about the sparkplug:
sparkplug insulator nose must have the self-cleaning temperature. The
self-cleaning temperature is determined by the heatrange of the sparkplug.
This is the temperature at which the carbon deposits that form on the nose
are burnt away. Too cold sparkplug heatrange won't burn away the
constantly forming carbon deposits, while too hot heatrange may not remove
the heat sufficiently and cause electrodes meltdown and other associated
sparkplug is an NGK B9EGV it was used for about 3 races on my racing ZIP
SP. The insulator nose is quite clean, that tells us that the heatrange
was properly selected. It recieved an
plugchop, so we can clearly see what we should look for; the mixture
ring. The mixture ring forms on the bottom of the insulator nose. It
tells us how the engine was jetted.
the mixture ring on the picture above, we can say that jetting was close
the jetting would be too rich, the mixture ring would thicken, scale up
towards the nose tip.
the jetting would be too lean, the mixture ring would thin down towards
the bottom, when the ring would become really thin or disappear, then we
could have big problems.
it's simple as that. However a much bigger problem is to properly see the
mixture ring without destroying the sparkplug each time we check the
jetting. An good eye with good sunlight may be enough to see inside, or
else lens with additional light should be used.
a daily used engine there is no need to sharply tune it, so jetting is
usually on the rich side to prevent it from having big problems, but if
the engine is raced, every bit of power should be extracted from it, so
jetting should be adapted to weather and track. These are some guidelines
to set the jetting:
dry (high or no clouds) - air pressure is high; bigger jets.
fog (low clouds, fog) - air pressure is low,humidity in air; smaller jets.
straight race track - bigger jets, colder sparkplug heatrange.
closed track - smaller jets, hotter sparkplug.